1) Field of the Invention
The present invention relates to an optical recording device that irradiates a laser beam on an optical recording medium to record information in the optical recording medium and an aberration correction method.
2) Description of the Related Art
When information is recorded in an optical recording medium like a laser disk (LD), a compact disk (CD), or a digital versatile disk (DVD), or when the information is reproduced from the optical recording medium, aberrations may occur such as spherical aberration, comatic aberration, or astigmatism. The spherical aberration is caused by fluctuations in thickness of a transparent substrate protecting the optical recording medium or by a fluctuation in the parameters of optical components. The comatic aberration is caused by warping of the optical recording medium, by fluctuation of parameters or adjustment deviation in optical components, or the like. The astigmatism is caused by deviation, inclination, or the like in accuracy of optical components, assembly errors, or deviations in optical axes. When a diameter of a laser beam on the optical recording medium increases due to these aberrations, correct information cannot be recorded in the optical recording medium, and recorded information cannot be reproduced correctly. Thus, various conventional techniques for correcting aberrations have been devised.
In a first conventional technique, taking notice of the fact that, when a spherical aberration occurs, a tracking servo gain decreases according to a degree of the spherical aberration, a spherical aberration correction value is transitioned such that the tracking servo gain increases. In other words, taking notice of the fact that a tracking error signal decreases when a spherical aberration is large and that a tracking signal increases when a spherical aberration is small, a spherical aberration correction value is transitioned. More specifically, from a disturbance superimposed signal, which is obtained by adding a disturbance signal (a signal having a predetermined frequency) to a tracking error signal and is used for control of a tracking servo system, only a band component of the disturbance signal is extracted to obtain a servo residual error value. Then, a ratio of the servo residual error value with respect to an amplitude of the disturbance signal is calculated, and this ratio is set as a tracking servo gain. When the optical recording medium rotates once, spherical aberration correction is performed by adding a predetermined value to a present spherical aberration correction value to calculate a tracking servo gain, and the calculated tracking servo gain and an immediately preceding tracking servo gain are compared. Then, a spherical aberration correction value with a larger tracking servo gain is determined as a new spherical aberration correction value (e.g., see Japanese Patent Application Laid-Open No. 2001-250256).
In a second conventional technique, taking notice of the fact that, when changes in levels are measured of a land pre-pit (LPP) signal, an RF (Radio Frequency) signal, a main push-pull (MPP) signal in an unrecorded state at the time when comatic aberration correction is operated at disk tilt 1 deg, and as a result of the measurement, when the comatic aberration correction is performed optimally, the levels of the respective signals take maximum values, and sensitivity of the signal levels is highest in the LPP signal and decreases in an order of the RF signal and the MPP signal. The RF signal is monitored in a recorded optical recording medium and ROM optical recording medium, and the LPP signal is monitored in an unrecorded DVD-R (Digital Versatile Disk Recordable) and DVD-RW (Digital Versatile Disk ReWritable) optical recording media to determine a comatic aberration correction value such that a signal amplitude of the monitored signal is maximized (e.g., see PIONEER R&D 2003 Vol. 13 “1. DVD-R/RW(R5) pickup development”).
However, a change in the tracking servo gain is small, and sensitivity thereof is low. In addition, tracking servo gains are different in an unrecorded optical recording medium and a recorded optical recording medium. Therefore, as an example, in the first conventional technique in which a spherical aberration correction value is determined such that a tracking servo gain is maximized, there is a problem in that a spherical aberration cannot be corrected with high accuracy.
Since the tracking servo gain changes subtly depending on a position of a recording medium, it is difficult to detect the subtle change accurately. Therefore, in the first conventional technique in which a spherical aberration correction value is determined such that a tracking servo gain is maximized, there is a problem in that spherical aberration correction according to a subtle change in the tracking servo gain cannot be performed. Although it is possible to detect a subtle change in the tracking servo gain if high-performance components are used, cost increases in this case.
Moreover, the tracking error signal provides information on only a tracking direction (radial direction). Therefore, in the first conventional technique in which a spherical aberration correction value is determined using a tacking servo gain that is calculated based on the tracking error signal, there is a problem in that correction cannot be performed of an aberration giving influence in a tangential direction.
In addition, as indicated in the second conventional technique, when a tilt correction amount by a liquid crystal element approaches “0,” signal changes decrease in the LPP signal, the RF signal, and the MPP signal. In other words, when the comatic aberration decreases and the tilt correction amount decreases, changes in signal amplitudes of the respective signals become flat. In the second conventional technique, taking notice of the sensitivity of a signal level, the LPP signal, which has a large change in a signal due to a correction amount for the comatic aberration is larger compared with the other signals, is used in the DVD-R and the DVD-RW in which the LPP signal is present. Further, the RF signal having a largest change next to the LPP signal is used in an optical recording medium in which the LPP signal is not present. However, in the case of any of the signals, the signal change decreases excessively when the correction amount of the comatic aberration decreases. Therefore, in the LPP signal, the RF signal, or the MPP signal adopted in the second conventional technique, there is a problem in that, when the tilt correction amount decreases, it is difficult to detect a change in a signal correctly, and an accurate comatic aberration correction value cannot be determined.